Fibril cellulose refers to isolated cellulose microfibrils or microfibril bundles derived from cellulose raw material. Fibril cellulose, which is also known as nanofibrillar cellulose (NFC) and by other related names, is based on a natural polymer that is abundant in nature. Fibril cellulose has many potential uses for example based on its capability of forming viscous gel in water (hydrogel).
Fibril cellulose production techniques are based on grinding (or homogenization) of aqueous dispersion of pulp fibers. The concentration of fibril cellulose in dispersions is typically very low, usually around 1-5%. After the grinding or homogenization process, the obtained fibril cellulose material is a dilute viscoelastic hydrogel. The material itself is usable as such in many applications, but logistic costs are too high to transport the material from the production site. In some applications, the high water content is not acceptable, i.e. the formulations do not tolerate large amounts of water.
EP 2 441 885 A1 discloses a process for producing modified cellulose fibers. A fibrillated cellulose dispersion having a concentration of 0.01% by weight of higher may be filtered to produce a cellulose sheet using vacuum or pressure filtration.
U.S. Pat. No. 5,964,983 discloses microfibrillated cellulose containing at least 80% of primary walls and loaded with carboxylic acids, and a method for preparing same. The process comprises the following steps:
(a) hydrolyzing the pulp with acid or base at a temperature between about 60° C. and 100° C. partially to extract pectins and hemicelluloses to form a suspension;
(b) recovering a solid residue from the suspension from step (a);
(c) carrying out, under alkaline conditions, a second extraction of the residue of cellulosic material from step (b), when step (a) is an acidic hydrolysis and recovering the cellulosic material residue by separating the suspension;
(d) washing the residue from step (b) or step (c);
(e) diluting the cellulosic material from step (d) in water to obtain between 2% and 10% dry matter;
(f) homogenizing the cell suspension from step (e) wherein
at least one alkaline extraction step is carried out on the cellulosic material, said alkaline extraction being carried out with a base at a concentration less than about 9% by weight, and
the homogenizing step (f) is carried out by a high mechanical shear operation to provide a cell suspension, passing the cell suspension through a small diameter orifice, subjecting the suspension to a pressure drop of at least 20 MPa and to a high velocity shearing action followed by a high velocity decelerating impact.
Strong water retention is typical for fibril cellulose since water is bound to the fibrils through numerous hydrogen bonds. For example reaching a dry substance (DS) content of higher than 10 wt-% by mechanical means is not economically possible in industrial scale. Conventional methods such as vacuum filtration or centrifugation are not suitable for fibril cellulose hydrogels. Thermal drying or any other methods are too expensive for liquid removal at this DS range.
The fundamental problem in mechanical water removal is the ability of fibril cellulose hydrogel to form a very dense and impermeable nanoscale membrane around itself, for example during filtration. The formed shell prevents diffusion of water from the gel structure, which leads to very slow concentration rates. The same applies to vacuum evaporation where the skin formation blocks the evaporation of water.
Another problem in drying of fibril cellulose is the non-redispersibility of the dried material. During the water removal, the fibril-water bonds are replaced with fibril-fibril interactions and the fibrils are permanently aggregated. This can be prevented with the use of certain additives during the drying stage, such as CMC, or by chemical modification of the microfibril surface, e.g. oxidation or carboxymethylation. With those methods fibril cellulose can be re-activated after complete drying.
Thus, there is an evident need for increasing the concentration of the final product so that the transport costs would be decreased and the fibril cellulose could be used in the final destination at a suitable concentration desired by the end user by simply redispersing the fibril cellulose in water. Further, there is a need for increasing the concentration of the fibril cellulose initially to remove the most of the water so that the further handling of the fibril cellulose is facilitated, regardless of the type of the process where the fibril cellulose is to be used.